1. Field of the Invention
This invention is related in general to edge strips for electrodes used in electrolytic processes such as electroforming, electrolytic extraction and refining of metals. In particular, the invention describes a padded edge strip with floating retaining pins that significantly reduce the deposition of material on the edges of electrode plates and around pin holes during the electrolytic process.
2. Description of the Related Art
Electrolysis is utilized to extract metals and other cations from electrolytic solutions. The extraction process is carried out by passing an electric current through an electrolyte solution of a metal of interest, such as copper, zinc, gold, silver, or lead. The metal is extracted by electrical deposition as a result of current flow between a large number of anode and cathode plates immersed in cells of a dedicated extraction tank house. Cathodes are generally constructed of a metal alloy, such as titanium or copper alloys and various grades of stainless steel resistant to corrosive acid solutions. In the most efficient processes, each cathode consists of a thin sheet of metal of uniform thickness (2-4 mm) disposed vertically between parallel sheets of anodic material, so that a uniform current density is produced throughout the surface of the cathode. A solution of metal-rich electrolyte and various other chemicals, as required to maintain an optimal rate of deposition, is circulated through the extraction cells. As an electrical current is passed through the anodes, electrolyte and cathodes, a pure layer of electrolyte metal is electro-deposited on the cathode surface, which becomes plated by the process.
The layer of pure metal so deposited is grown to a specific thickness on the cathode during a predetermined period of process time and then the cathode is removed from the cell. It is important that the layer of metal deposited be recovered in uniform shapes and thicknesses and that its grade be of the highest quality, so that it will adhere to the cathode blank during deposition and be easily removed by automated stripping equipment afterwards. The overall economy of the production process depends in part on the ability to mechanically strip the cathodes of the metal deposits at high throughputs and speeds without utilizing manual or physical intervention. To that end, the cathode blanks must have a surface finish that is resistant to the corrosive solution of the tank house and must be strong enough to withstand their continuous handling by automated machines without pitting or marking. Typically, the stripping process involves a step during which the cathode plates are slightly bent to cause the product layer to become separated. Any degradation of the blank's finish causes the electro-deposited metal to bond with the cathode resulting in difficulty of removal and/or contamination of the deposited metal.
It is very important that metal deposition be avoided along the edges of the electrodes to prevent the formation of a continuous layer of deposit between opposite sides of the plate which would complicate and delay the stripping process. Thus, in order to prevent electrolyte build-up along the double-sided edges of the starter sheet, which would impede the automated separation of the product at the end of each cycle, these edges are masked with an insulating strip fastened to the electrode. Such edge strips are designed to tightly wrap around the edges of the starter sheet and prevent deposition of material past the line of contact between the strip and the starter sheet. In order to improve the uninterrupted contact between the strip and the starter sheet, several kinds of edge strips have been developed with different advantages best suited to specific applications.
For example, U.S. Pat. No. 4,406,769 to Berger (1983) discloses an edge protector consisting of a strip having an H-shaped cross-section so as to provide open slots on opposite sides. One slot is defined between a pair of parallel jaws and is adapted for receiving the edge of an electrode; the other slot is substantially semicircular and is adapted to receive a tubular member in compression, so that its insertion results in a leveraged narrowing of the first slot and a corresponding tight frictional connection between the edge strip and the cathode.
In U.S. Pat. No. 4,776,928 (1988), Perlich describes a co-extruded structure for an edge protector consisting of a rigid U-shaped member having parallel jaws that define a slot for receiving the edge of an electrode and a pair of resilient lips attached to the ends of the jaws to press tightly against the electrode edge, thereby impeding penetration of electrolyte. This patent first disclosed the concept of using dual-durometer co-extruded members to improve gripping of the edge strip to the electrode surface.
In U.S. Pat. No. 5,314,600 (1994), Webb et al. introduced the concept of including a longitudinal groove within the edge slot for accommodating and engaging transverse pins protruding from the electrode. This edge protector also includes expansion channels to facilitate the insertion of the electrode's edge into the protector's slot.
Finally, U.S. Pat. No. 5,549,801 to Perlich (1996) describes an edge protector that includes a resilient hinge for improving the adherence of the strip to the edge of the electrode. An expansion member is also provided to secure the edge strip firmly in place.
Problems remain in the art due to the fact that edge strips need to be sufficiently rigid to retain their shape over severe temperature cycles and maintain continuity of contact with opposite surfaces of the starter sheet's double-sided edges. Since the compressive force exerted by the strip on the edge depends on the resilience of the strip's material, the tightness of the connection tends to diminish as the material ages and deteriorates through thousands of deposition and stripping cycles. It is critical that a sufficient degree of compression on the edge of the electrode plate be present at all times to prevent penetration of the electrolyte solution during the deposition process. If the material is too rigid, the edge strip's performance becomes very dependent on a perfect fit of the starter sheet within the strip's edge slot; if too resilient, the strip may more easily conform to variations in smoothness and thickness in the starter sheet but it may also be easily deformed by shocks and buckling forces, which all result in electrolyte solution penetration and material deposition within the strip's boundary.
Another serious problem derives from the fact that edge strips are fastened to the cathode through pins placed tightly through the plate's edge in transverse perforations disposed along the length of the strip. As a result of the bending to which the plate and strip are subjected during the stripping process, lateral stresses are necessarily imposed on the pins and tend to wear out both the pins and the perforations in the edge strip, thereby producing gaps that expose the metallic edge of the cathode plate to the electrolyte solution in which the cathode is immersed. This in turn produces the deposition of electrolyte in the gaps and the formation of very undesirable nodules around the pins and in the perforations.
In practice, once edge strips begin to have a less than perfect fit over the edge of a plate as a result of fatigue or damage, the electro-deposition metal penetrates through any gap and forms irregular deposits that make it difficult to strip the product from the plates. As a result, there is a tendency in an industrial environment to abuse the plates during the stripping process by bending them more than normal and resorting to hammering or other undesirable practice to remove infiltrated material, which causes further damage to the edge strips and in turn more undesirable build-up. This cycle of events rapidly destroys the effectiveness of normal edge strips and compels their premature replacement.
Therefore, there still exists a need for an improved electrode edge protector. The present invention is based on a novel construction approach that greatly reduces infiltration of electro-deposited material.